Graphics or digitizer tablets are well known in the art. See, for example, the January 1989 issue of BYTE magazine, pages 162-174. The pointing device typically used is a pen or stylus, or a cursor, also called a puck. Puck will be used herein to avoid confusion with a display or CRT screen cursor. The puck frequently comes equipped with buttons connected to switches. The tablet is usually connected to a computer with a display, and the puck plugs into the tablet. In a typical application, when activated, as the puck operated by the user follows, for example, a line drawing, the puck X-Y coordinates outputted to the computer are processed and cause the screen cursor to follow the puck, so that a replica of the line drawing is produced on the screen. The puck buttons often serve control or command functions, such as telling the tablet when to start and stop outputting X-Y coordinates, or when to recognize a valid puck position.
It is also known to convert stylus pressure as applied by the user into a control function for the tablet or computer. See, for example, U.S. Pat. Nos. 4,806,707 and 4,687,885. In the former, disposed within the pen body is an optical linear position transducer acted on by the pen tip to generate a Z-axis signal proportional to linear movement of the pen tip. In the latter patent, a Z-axis signal is generated by building into the tablet special resistive coatings determining the signal value as a function of the user's pressure. Both of the mechanisms described in these patents suffer from the problem that the Z-axis signal depends upon the ability of the user to manipulate the pen and apply the necessary pressure to achieve a desired Z-axis signal output. Moreover, the Z-axis signal is typically outputted either once, or continuously, provided that the user maintains continuously the pressure.
Reference is also made to commonly-assigned U.S. Pat. No. 4,786,764, which describes a stylus with a pressure transducer in the form of an ink layer having electrical resistance, providing an analog Z-axis output signal as a function of user pressure.